The present invention relates to an electronic display system for visually displaying map data stored, or more in particular to an electronic display system suitable for use on an automobile or the like.
Conventionally, a device called an electronic map display system has been used. Such device allows a necessary map to be rapidly searched for, and is used by an automobile driver who is lost, to enable him to reach his destination.
In such a system, required map data on given areas are stored in a large-capacity memory means, such as a magnetic disc or optical disc. By designating the area around the automobile with a select switch or the like, the map thereof is displayed on CRT or the like.
The data memory system for this display is generally limited to the image pattern system in which the image of a map is stored as is. A map image, for example, is subdivided and each subdivided unit is stored as an item of digital information in an arrangement corresponding to the map coordinates thereof.
The image pattern system, however, has the disadvantages of an increased amount of stored data and the fixed form of expression, thereby making it an unsuitable application for enlarged or shrunken expression. For this reason, a data storage system of structured type in place of the image pattern system is under development.
In the structured data storage system, each element of a map is expressed in a predetermined coordinate system. A road, for instance, is expressed in a rectangular coordinate system as points at regular intervals of distance.
The shrinking process of structured data, however, sometimes results in an inaccurate display as a map.
In the shrinking process for a road, for instance, all the data making up the road are not used. Typically, one half or fewer of the points are used, so that the coordinates of these points are connected and displayed. A possible result can be seen when it is intended to shrink a detailed picture like FIG. 13a to the one shown in FIG. 13b with fewer data (r1, r2). For instance, the point PO, which may be a filling station on the roadside and located accurately on the coordinate system as a whole, may be inconveniently relocated to the opposite side of the road by the shrinking process.
If a mark or target is relocated to the opposite side of a road on the display because of the magnification rate as discussed above, a great inconvenience is imposed on the vehicle driver looking for a mark to reach it. The driver, almost unable to closely watch both sides of a road while at the wheel, may overlook the mark.
In a similar manner, a mark distant from a road, such as a golf course, is relocated to a point different in distance from the road. The resultant inaccurate relative distance from the road in a displayed map is likely to cause the driver to make an unnecessary detour.